Repetitive exercise has been found to produce an increase in the size of the coronary vascular bed. The enlarged vascular capacity may furnish protection to the heart that is further stressed by a pressure overload by aortic constriction. Pressure overload will stimulate a cardiac enlargement that has been shown to result in heart failure. The physiological, biochemical, and ultrastructural changes in repetitive exercise followed by pressure overload will be studied in the chronic instrumented dog. The dogs will be studied during control periods and at intervals following aortic constriction. The points of comparison will be chosen from the degree of enlargement during the course of experiments. Physiological data will be obtained from the intact animal during an exercise test, cardiac pacing, and short coronary artery occlusions (hyperemic response). The biochemical function of the mitochondria, sarcoplasmic reticulum, lysosome, and myofibrillar ATPase will be studied at the conclusion of the experiment in these same animals. The ultrastructural changes in the cardiac muscle will be correlated with the biochemical and physiological responses. The localization of certain specific ions (Ca ions, Mg ions, and Cu ions) will be ascertained using a selective ion probe analysis coordinated with a scanning electron microscope. The transmission and scanning electron microscope will be used to determine cell membrane to myofibril ratio changes and change in the surface characteristics of the muscle cell. Any beneficial effect of repetitive exercise should change the relationship between the degree of enlargement and functional capacity of the heart.